Method and apparatus for estimating corrosion rate



July 27, 1965 G. A. MARSH ETAL 3,197,388

METHOD AND APPARATUS FOR ESTIMATING CORROSION RATE Filed Dec. 22, 3.961

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76.3 i ww/@ United States Patent O 3,197,388 METHOD AND APPARATUS FOR ESTIMATING CORROSION RATE Glenn A. Marsh and Robert L. Littler, Crystal Lake, lill., assignors to The Pure Oil Company, Chicago, Ill., a corporation of Ohio Filed Dec. 22, 1961, Ser. No. 161,449 6 Claims. (Cl. 204-1) This invention relates to a method and apparatus for determining the rate of corrosion of corrodible elements which form a lmulti-metallic, electrolytic corrosion system.

It has long been recognized that the rate of corrosion of one of a pair of dissimilar, corrodible metallic structures contacted by an electrolyte can be determined by measuring the zero-resistance current flow between the two structures. By application of Faradays Law, the measured current can be converted to a rate of metal loss, expressed in inches of penetration per year, or other conventional units. Many more complex corrosion systems include a relatively large number of structures, all fabricated of diierent metals, some or all of which are electrically connected, and all or which are contacted by an electrolyte. The study of corrosion rates in such complex systems has, in the past, been approached by exposing individual couples to the electrolyte, so that the behavior of selected structures of the system with respect to each other may be evaluated. This method has proved to be less than satisfactory, because it fails to take into consideration the interaction of the numerous couples which comprise a complex electrolytic corrosion system, and the effect which such couples have upon each other.

The deficiencies of the methods of the prior art have been found to be especially apparent when it is desired to determine the eciency of inhibitors used in complex electrolytic corrosion systems. For example, a gasoline engine and cooling system commonly will include members fabricated from cast iron, steel, copper, brass, solder, and aluminum, all of which are in Contact with a cooling fluid which is an electrolyte. All of the aforenamed metals are connected in substantially short-circuit electrical relationship. Evaluating the rate of corrosion of one metal structure in the system, say an aluminum cylinder head, cannot very well be accomplished by determining the corrosion rates of aluminum elements disposed in a plurality of individual couples, including as second elements of the couples elements fabricated of various other metals of the system, due to the fact that interaction of the cells is not thereby taken into consideration. The evaluation of corrosion inhibitors by determining their eifect upon various couples composed of selected metals ot the system is highly unreliable, because it fails to take into consideration the effect which the passivation of one couple may have upon another couple. For example, the corrosion of an aluminum cylinder head may serve to passivate the cathodic tendencies of a copper radiator tube with respect to an iron engine member. Hence the use of a corrosion inhibitor which is selective tor the passivation of aluminum, but not for iron, may actually result in a dramatic increase in the rate of corrosion of the iron member, and rapid failure thereof.

It is thereof an object of this invention to provide a method and apparatus for determining the rates of corrosion of members f multi metallic-electrolytic corrosion systems. Another object of this invention is to provide a method and apparatus for determining the rate of corrosion of members of complex electrolytic corrosion systems, whereby the interaction of the various couples which comprise the system is taken into consideration.

The invention is best described with reference to the drawings, of which,

3,197,338 Patented July 27, 1955 ICC FIGURE 1 is a sectional view of a corrosion-measuring probe fabricated in accordance with this invention,

FJIGURE 2 is a sectional View of an alternate corrosion probe, and

FIGURE 3 is a schematic diagram of an electric circuit which can be employed in conjunction with probes of FIGURES 1 and 2.

Referring to FIGURE l, rod 10, which is fabricated of an electrically insulating, corrosion-impervious material, such as polyethylene, is threaded to receive support at 12 and is encompassed by specimens 14, 16, 18, and Ztl, which are spaced axially along rod 10. Electrical conductors 22, 24, 26, and 2S extend through rod 10 in electrically insulated relationship, and make connection to conductors 14, 16, 13, and 20, respectively. The junction between each conductor and specimen is preferably at the inside of the opening provided in the specimen, so that the connection is protected from exposure to the environment surrounding the assembly. Each specimen is fabricated of a different metal, and is made to provide an appropriate surface area. For example, where it is desired to fabricate an apparatus for simualting the conditions which exist in a gasoline engine and cooling system assembly, comprising a copper tube radiator of soldered construction, an iron engine block, and an aluminum cylinder head, the specimen 14 may be made of copper, the specimen 16 of solder, the specimen 18 of iron, and the specimen 2li of aluminum. The surface areas of the specimens are proportioned to the surface areas of the corresponding engine elements which are in contact with the electrolyte, i.e., the cooling fluid. For example, if the surfaces of the engine block in contact with the cooling fluid have live times the area ofthe surface of cylinder head in contact with the cooling fluid, the surface area of specimen 18 can be made 5 times as great as the surface area of specimen 20. For this purpose, the specimen 1S may be constructed with a plurality of tins, as shown.

In operation, the assembly of FIGURE 1 is immersed in exposure to an electrolyte, for example in exposure to a sample of cooling fluid taken from the cooling system of an internal combustion engine. By connecting the conductors 22, 24, and 26 together, and measuring the short-circuit current flow between conductor 28 and the connected conductors, as by placing a low-resistance ammeter therebetween, the rate of corrosion of the aluminum specimen can be measured, and thereby the rate of corrosion of the aluminum cylinder head estimated. Similarly, the rate of corrosion of the engine block can be estimated by connecting together conductors 22, 24, and 23, and placing an ammeter between conductor 26 and the connected conductors. In this manner, the rate of corrosion of each specimen, when under the influence of the corrosive environment and the effects of the remaining specimens, can be ascertained.

Referring to FIGURE 2, an alternate structure is shown. Block Si) is fabricated of an electrically insulating and corrosion-impervious material, such as a polyester resin. Tubular specimens 32, 34, and 36 are supported by base 3i). Conductors 38, di), and 42 pass through the block 3) and connect ends of the tubular specimens 32, 34, and 3o, respectively.

Referring to FIGURE 3, a convenient switching and measuring arrangement for use in conjunction with abovedisclosed assemblies is shown. Connection is made from the conductors of the test assemblies to the terminals, such as terminals 54B, of switch assembly S2. The switch assembly includes a base 54, and a slide bar 56, both of which are made of an electrically insulating material. Conductive contact bars 5S, oil, and 62 are attached to the slide bar 56. Contact 53 is connected to measuring circuit terminal 64, while contacts 60 and 62 are joined sa together and connected to measuring-circuit terminal 65. Between the terminals ed and 66, a variable resistance, a battery, and an ammeter are series-connected. A voltineter is also connected across terminals 6ft and e5. This circuit provides a very convenient means for measuring the zero-resistance current output of the specimens of the probe. The function of the variable resistance and the battery is to overcome the resistance of the ammeter measuring circuit. This is accomplished by setting the variable resistance so that the reading on the sensitive voltmeter is zero. In this situation, there is zero voltage drop across the ammeter measuring circuit. By moving slide 5d to the right or to the left, the short-circuit current between any selected specimen and the remaining specimens can be conveniently measured.

As a specific example of the method of this invention, and the use of the apparatus of this invention, it is desired to estimate the rate of corrosion of the aluminum member of a multi-metallic structure fabricated of four different metals, including aluminum, all of which are exposed to a corrosive environment. The corrosive environment comprises an aqueous salt solution. Four metal specimens are fabricated of metals identical to those which comprise the multi-metallic structure. The ratio of the area of the portion of the structure fabricated of a particular metal, and exposed to the corrosive environment, to the area of the specimen fabricated of the same metal is made constant for each portion of structure and corresponding specimen. This is done by adjusting the specimens to provide the required surface areas. The specimens are then suspended in a sample of the electrolyte, and electric lead Wires, which themselves are insulated from the environment, are connected to each specimen. The lead wires coming from the various specimens, excluding that of aluminum, are connected together, and a zero-resistance ammeter is employed to measure the shortcircuit Icurrent flow between the aluminum specimen and the other connected specimens. A current reading of 55 milliamperes is obtained. All of the lead wires except that connected to the steel specimen are then connected together, and the current ow between the steel specimen and the remaining specimens is measured and found to amount to 5 milliamperes. ln each case, the direction of current flow is such as to indicate a corrosion of the aluminum and steel specimens. A corrosion inhibitor is then aded to the electrolyte in which tre specimens are immersed. The measurements are repeated with respect to the aluminum specimen and the steel specimen, and readings of 6 milliamperes and 8 milliamperes, respectively, are noted. From the foregoing tests, it is concluded that the inhibitor employed is elfective to mitigate the corrosion of aluminum in the system under study, but that the inhibitor can be used only if a rate of corrosion of the steel corresponding to the eight milliampere current can be tolerated.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. The method of estimating the rate of corrosion of a selected corrodible structural member in a multi-metallic, electrolyte-containing corrosion system including the `selected member and a plurality of other electrically connected members fabricated of diverse metals comprising,

(a) exposing a specimen fabricated of the same metal l as said selected member to a body of electrolyte substantially identical with that of said corrosion system,

(b) exposing a plurality of other specimens each of which is fabricated of the same metal, respectively, as one of the other members of said plurality to said body of electrolyte, each of all said specimens having a ratio of its surface area to the surface areas of the other specimens in contact with said body of electrolyte the same as the corresponding ratio of cach member to the other members in contact with electrolyte in said corrosion system so as to provide a proportional duplicate of said corrosion system.

(c) electrically connecting the specimens of said plurality in short circuit, and

(d) measuring the Zero resistance current flow between said first specimen and the specimens of said plurality connected in short circuit.

2. A corrosion-measuring apparatus comprising,

(a) at least three corrodible specimens fabricated of different metals,

(b) means for supporting said specimens in electrically insulated relationship for exposure to a corrosive electrolyte,

(c) three electrical conductors connected to said three specimens, respectively,

(d) electrical switching means comprising at least three terminals to each of which is fastened one of said conductors,

(e) an electric circuit comprising in series an ammetcr, a direct current source and a variable resistance and a voltmeter connected in parallel therewith,

(f) at least three separate electrical contact elements movable into and out of contact with said terminals,

(g) means connecting at least two of said contact elements in short circuit to one terminal of said circuit, and

(h) means simultaneously connecting a third contact means to the other terminal of said circuit.

3. An apparatus in accordance with claim 2 in which aid means for supporting the specimens is fabricated of a corrosion-impervious, electrically insulating material.

d. An apparatus in accordance with claim 3 in which said support means comprises a rod adapted for insertion in a corrosive environment, and said specimens are spaced axially along the rod.

5. An apparatus in accordance with claim I in which said conductors extend within the rod from one end thereof to contact said specimens.

6. An apparatus in acordance with claim 5 in which each specimen is generally disc-shaped and provided with an opening through which the rod extends to internally contact and support the specimen.

References Cited by the Examiner UNTED STATES PATENTS '2,805,191 9/57 Hersch 21M-1.1 2,903,405 9/57 Sabios 2041-197 3,050,371 8/62 Dowson et al 204-1.1 3,051,631 8/62 l-larpin et al 284-195 WINSTON A. BOUGLAS, Prnmry Examiner.

JOHN R. SPECK, Examiner, 

1. THE METHOD OF ESTIMATING THE RATE OF CORROSION OF A SELECTED CORRODIBLE STRUCTURAL MEMBER IN A MULTI-METALLIC, ELECTROLYTE-CONTINING CORROSION SYSTEM INCLUDING THE SELECTED MEMBER AND A PLURALITY OF OTHER ELECTRICALLY CONNECTED MEMBERS FABRICATED OF DIVERSE METALS COMPRISING, (A) EXPOSING A SPECIMEN FABRICATED OF THE SAME METAL AS SAID SELECTED MEMBER TO A BODY OF ELECTROLYTE SUBSTANTIALLY IDENTICAL WITH THAT OF SAID CORROSION SYSTEM, (B) EXPOSING A PLURALITY OF OTHER SPECIMENS EACH OF WHICH IS FABRICATED OF THE SAME METAL, RESPECTIVELY, AS ON OF THE OTHER MEMBERS OF SAID PLURALITY TO SAID BODY OF ELECTROLYTE, EACH OF ALL SAID SPECIMENS HAVING A RATIO OF ITS SURFACE AREA TO THE SURFACE AREAS OF THE OTHER SPECIMENS IN CONTACT WITH SAID BODY OF ELECTROLYTE THE SAME AS THE CORRESPONDING RATIO OF EACH MEMBER TO THE OTHER MEMBERS IN CONTACT WITH ELECTROLYTE IN SAID CORROSION SYSTEM SO AS TO PROVIDE A PROPORTIONAL DUPLICATE OF SAID CORROSION SYSTEM. (C) ELECTRICALLY CONNECTING THE SPECIMENS OF SAID PLURALITY IN SHORT CIRCUIT, AND (D) MEASURING THE ZERO RESISTANCE CURRENT FLOW BETWEEN SAID FIRST SPECIMEN AND THE SPECIMENS OF SAID PLURALITY CONNECTED IN SHORT CIRCUIT. 